Magnetic pigments and process of enhancing magnetic properties

ABSTRACT

A pearlescent pigment and a process for enhancing the magnetic properties of a pearlescent pigment. The pearlescent pigment has a layer with regions of γ-Fe 2 O 3  and regions of α-Fe 2 O 3 . The magnetic properties of a pearlescent pigment may be enhanced by the steps of: providing a platelet pigment with a layer of Fe 2 O 3 , with a magnetic susceptibility less than 0.1×10 −5  m 3 /kg; reducing some or all of the Fe 2 O 3  to Fe 3 O 4 ; and oxidizing some or all of the Fe 3 O 4  to γ-Fe 2 O 3 . The color difference (ΔE*) between the provided pigment and the resultant pigment is not more than about 5.

BACKGROUND

Along with gem stones (e.g., diamond, ruby, emerald, topaz, opal, jade),and precious metals (e.g., gold, silver, platinum), pearls are among themost prized possessions (or luxury items) for human beings formillenniums. Beside their natural beauty, the brilliant color andluster, they are often associated with social status and level ofwell-being. As a result, and not surprisingly, the trend of cosmeticsmakeup is to emulate or recreate these “natural” and “aesthetic”appearances of pearl, gem and precious metals with less expensivematerials such as interference pigments (e.g., metal oxide coated mica).The most common types of effect pigments are micronized titaniumdioxide, metal oxide coated mica, metal oxide coated alumina, metaloxide coated silica, basic lead carbonate, bismuth oxychloride, andnatural fish silver.

Metal oxide coated mica pigments are characterized by excellent optical,chemical, mechanical, toxicological, and environmental properties.Natural or synthetic mica, and alternative supports, such as aluminumflakes, or SiO₂ platelets, can be used alone, or as a support fortitanium dioxide, iron oxide (Fe₂O₃ or Fe₃O₄), iron ferrocyanide (IronBlue or Prussian Blue), tin oxide, and chromium oxide. The color spacedefined by these coated mica-based pigments is based on the type ofcoating (e.g. metal oxide, colorant, etc.) used, the layer thickness,and the number of coated layers.

Among the natural pearls, the most expensive are black pearls, whichcome with various undertone and color flops. To faithfully emulate thisaesthetic optical effect in cosmetic makeup is one of the top challengesfacing a cosmetic pigment maker and formulator. The traditional approachto these pigments is to blend dark solid-color inorganic pigment (e.g.,black iron oxide or carbon black) with white platy pearlescent pigments(e.g., TiO₂ coated mica, TiO₂ coated borosilicate, TiO₂ coated alumina).The platy interference pigment provides the luster, brilliance(reflection), transparency and depth of field. The solid-colorpigment(s) provide(s) the dark undertone and surface coverage. However,this type of blend usually appears to be much “dirtier”, “lack luster”,and “lack transparency” compared to the natural pearl. The primaryreason for that is fouling of the smooth surface of white pearlescentpigment by the solid-color pigment granules, which leads to lightscattering and disruption of light interference.

Metal oxide coated platelet pigments may be magnetic or exhibit magneticsusceptibility. When placed into a liquid coating, regions of the coatedpigment may be aligned by an externally applied magnetic field andproduce a goniochromatic, or angle dependent optical effect. This effectmay be used to create an impression of a two- or three-dimensionalimage. After the pigments have been aligned, the coating may be cured tosolidify the optical effect. Examples of pigments and methods ofaligning them are discussed in U.S. Pat. Nos. 6,589,331; 6,902,807;5,223,360; 6,759,097; and 7,258,900.

The use of metallic colored (copper, bronze, maroon/russet shades, gold,etc.) pearlescent pigments is widespread and can be applied in fieldssuch as decorative cosmetics, plastics, advanced security printing andautomotive and industrial coatings. Commercial products currentlyavailable include: Iriodin®, Xirallic®, Timiron®, Xirona® and Colorona®lines by Merck, Cloisonné® and Timica® lines by BASF, SunPearl® andSunshine® lines by SunChemical. Metallic pearlescent shades (bronze,copper, russet, etc.) contained within these lines of pigments aregenerally developed by deposition of α-Fe₂O₃ (hematite) on the surfaceof a platelet-like substrate, such as mica, Al₂O₃ platelets, calciumborosilicate, or other laminar substrates. Single-layer pigments of thistype have a yellow-red absorbance color combined with an interferencecolor directly related to the thickness of the iron oxide layer. Thiscombination of light reflection, absorption and interference can beutilized to produce lustrous, pearlescent effects ranging from gold todeep maroon shades. In addition, given that iron oxides absorb a portionof the incident light, these pigments are defined by intermediate hidingpower relative to transparent TiO₂-coated pearlescent pigments andopaque metallic effect pigments (such as aluminum flake).

Pearlescent pigments comprised of a platelet-like substrate coated withα-Fe₂O₃ generally have magnetic mass susceptibility values in thevicinity of 0.01 to 0.02×10⁻⁵ m³/kg. Thus, these pigments are not easilyapplicable to printing methodologies that utilize an external magneticfield to manipulate pigment orientation such as those described in U.S.Pat. Nos. 5,223,360; 6,645,286; and 6,759,097. Platelet-like substratescoated with Fe₃O₄ (magnetite) are defined by a much higher magneticsusceptibility. For instance, Colorona® Blackstar Red, and BlackstarGold have magnetic mass susceptibility values of 11.56 and 11.08×10⁻⁵m³/kg, respectively. These types of pigments can be used inmagnetically-aligned coating applications; however, they are generallyconfined within a very narrow color space (dark shades or dark shadeswith muted inference colors).

Consequently, a significant need exists for pigments with highermagnetic susceptibility, more colors, and are easier to manufacture.

BRIEF SUMMARY

The above-noted and other deficiencies may be overcome by providing apigment comprising a substrate and a layer, wherein the layer hasregions of γ-Fe₂O₃ and regions of α-Fe₂O₃.

Magnetic properties of a pigment may be enhanced by the steps of:providing a platelet pigment with a layer of Fe₂O₃, with a magneticsusceptibility less than 0.1×10⁻⁵ m³/kg; reducing some or all of theFe₂O₃ to Fe₃O₄; and oxidizing some or all of the Fe₃O₄ to γ-Fe₂O₃.

In one embodiment, a process for making a pigment comprises the stepsof: providing a platelet pigment; increasing the magnetic masssusceptibility;

wherein the color difference (ΔE*) between the provided pigment and theresultant pigment is not more than about 5.

In another embodiment, a pigment may be formed by the steps comprising:providing a platelet pigment; increasing the magnetic masssusceptibility; wherein the color difference (ΔE*) between the providedpigment and the resultant pigment is not more than about 5.

These and other objects and advantages shall be made apparent from theaccompanying drawings and the description thereof.

DETAILED DESCRIPTION

A process for enhancing the magnetic properties of pigments comprisesthe steps of providing a platelet pigment with a layer of Fe₂O₃, with amagnetic susceptibility less than 0.1×10⁻⁵ m³/kg; reducing some or allof the Fe₂O₃ to Fe₃O₄; and oxidizing some or all of the Fe₃O₄ toγ-Fe₂O₃.

In one embodiment, a pearlescent pigment comprises a substrate and alayer, wherein the layer has regions of γ-Fe₂O₃ and α-Fe₂O₃. In oneembodiment, the region of γ-Fe₂O₃ is further from the substrate than theregion of α-Fe₂O₃. Iron oxide coated substrates exhibit intenselycolored pearlescent pigments with high luster. Varying the substrate andthe iron oxide layer thickness may change the color, luminosity, andtransparency of the pigment. The mean thickness of the first layer maybe from about 1 nm to about 350 nm, from about 10 nm to about 350 nm, orfrom about 10 nm to about 250 nm.

In one embodiment, the pigment may comprise a second layer locatedbetween the substrate and the first layer, wherein the second layer hasa refractive index of greater than about 1.6 or less than about 1.4. Thesecond layer may have a refractive index equal to or greater than about1.8. In one embodiment the second layer comprises: TiO₂, Fe₂O₃, FeOOH,ZrO₂, SnO₂, Cr₂O₃, BiOCl, and ZnO. The second layer may comprise one ormore materials. The second layer may be TiO₂. The second layer may be aniron oxide, such as Fe₂O₃, Fe₃O₄, FeOOH, FeO, and Fe(OH)₃. The meanthickness of the second layer may be from about 50 nm to about 800 nm,or from about 100 nm to about 600 nm.

The synthesis of a particular colored pearlescent pigment begins withselection of the proper substrate material. The substrate may becomprised of natural mica, synthetic mica, glass flakes, metal flakes,talc, kaolin, Al₂O₃ platelets, SiO₂ platelets, TiO₂ platelets, graphiteplatelet, BiOCl, calcium borosilicate, synthetic alumina, and boronnitride. Examples of glass flakes are borosilicate. Glass flakes areprimarily composed of SiO₂ and Al₂O₃ and can also include ZnO, CaO,B₂O₃, Na₂O and K₂O as well as FeO and Fe₂O₃. Examples of metal flakesare aluminum, copper, zinc, and other metals and alloys havingmalleability. Examples of other metals and alloys having malleabilityare nickel, magnesium, aluminum-copper alloy, aluminum-zinc alloy,aluminum-nickel alloy, and aluminum-magnesium alloy. Metal flakes may beused alone or in any combination thereof. Substrates may be multilayermaterials, i.e. include materials of different refractive indices. Thesubstrate may comprise mica. The pearlescent pigment may comprise amixture of different substrates. The substrate may be made of identicalor different flakes which differ in particle size. Other examples ofsubstrates are those that are fibers. Examples of fiber substrates arecarbon fiber, glass fiber, and polymeric fibers.

In one embodiment, the substrate is platelet-like and may have a meanthickness of about 0.05 to about 1.5 μm and a mean width of about 1 toabout 750 μm. The substrate may have a mean width of about 10 to about60 μm, about 5 to about 25 μm, about 10 to about 100 μm, about 40 toabout 250 μm, or about 95 to about 730 μm.

The preparation of α-Fe₂O₃ coated platelet-like substrates is wellknown. In general, deposition of α-Fe₂O₃ surface layers is achieved byprecipitation of FeOOH or various modifications of FeOOH followed byannealing at temperatures ranging between 400 to 1100° C. This processis described in Dyes and Pigments, 58 (2003), 239-244, and U.S. Pat.Nos. 3,926,659; 3,087,829; and 3,926,659. The color associated withα-Fe₂O₃ coated platelet-like pigments is determined by the interplay oflight interference and absorption. Through precise control of theα-Fe₂O₃ layer, lustrous metallic effects can be produced ranging frombronze-yellow to deep red.

Hematite (α-Fe₂O₃) is a weakly, magnetic iron oxide. Natural mica-basedpigments containing layers of α-Fe₂O₃ generally have magnetic masssusceptibility values ranging from about 0.01 to 0.02×10⁻⁵ m³/kg.Increasing the magnetic mass susceptibility of pearlescent pigments suchas these while maintaining their popular and attractive colors willallow for manipulation of the platelet-orientation in uncured coatingsor liquid-based suspensions via an applied external magnetic field,expanding the available color space available for magnetically alignedcoatings.

The magnetic mass susceptibility may be enhanced by partial reduction ofthe α-Fe₂O₃ followed by oxidation to γ-Fe₂O₃. Reduction may beaccomplished by many methods such as NaBH₄, calcining in a reducingatmosphere, or homogeneous hydrogenation of a suspended solids solutionin the presence of a precious metal catalyst. Examples of homogeneoushydrogenation have been described in U.S. Ser. No. 11/931,534 which ishereby incorporated by reference in its entirety.

During hydrogenation of suspended solids solutions, sufficient contactbetween the α-Fe₂O₃ coated substrate and catalyst may be achieved if thecatalyst and suspending solvent behave as a single homogeneous phase. Asthe catalyst particle size approaches the nanoscale (1 to 30 nm), themass transfer limitations inherent to heterogeneous catalysts aremitigated due to a very high catalyst surface area to volume ratio.

Agitation influences reduction because it controls the interfacial areafor hydrogen transport into the solvent and prevents substrate settling.Although increased agitation will generally result in improved reductionrates, it may also result in a loss of pigment luster due to pigmentfragmentation (particularly for particles greater than 60 μm). Increasedcatalyst loading or time will result in further reduction of the Fe(III)surface layer and higher magnetic mass susceptibility.

Reduction converts Fe₂O₃ to Fe₃O₄ that when oxidized to γ-Fe₂O₃ will notchange the color of the final pigment significantly relative to thestarting substrate. After reduction, the pigment produced issignificantly darker than its corresponding starting substrate.Dependent on the level of reduction, the magnetic mass susceptibility ofthe starting substrate can be increased by up to 3 orders of magnitude.

In order to restore the original color of the starting substrate, thehydrogenated pigment is oxidized. One method of oxidizing the iron oxideis by calcination at temperatures greater than about 350° C. Thecalcination may be at a temperature ranging from 400 to 1100° C. Thecalcination may be in an oxidizing atmosphere, such as air. Calcinationtransforms the outer magnetite layer into maghemite (γ-Fe₂O₃), anotherhighly magnetic iron oxide, thus yielding a magnetic pigment withrelatively equivalent color (ΔE* less than about 1) compared to thestarting pigment.

Following re-oxidation, the overall amount of Fe₂O₃ contained on thepigment is nearly equivalent. The only change is the form of the Fe₂O₃(i.e., mica+α-Fe₂O₃ is converted to mica+α-Fe₂O₃+γ-Fe₂O₃) which resultsin minimal color change but dramatic change in the magnetic masssusceptibility. In one embodiment, the ratio of α-Fe₂O₃ to γ-Fe₂O₃ isbetween about 0.05 and about 50. Thus, metallic shade pearlescentpigments may be transformed into magnetic pigments significantlyincreasing the color space available for magnetically-aligned coatingapplications. In one embodiment, the color difference (ΔE*) of thepigment prior to reduction and after oxidation is not more than about 5.In one embodiment, the pigment has a magnetic susceptibility of about0.1×10⁻⁵ to 15×10⁻⁵ m³/kg.

In one embodiment the pigment may have an additional outer layer on thefirst layer. The outer layer may comprise a metal oxide. Examples ofmetal oxides are TiO₂, Fe₂O₃, FeOOH, ZrO₂, SnO₂, Cr₂O₃, BiOCl, and ZnO.The outer layer may alter the color of the pigment so it is no longersimilar to the color of the pigment prior to reduction.

In one embodiment, the process for making a pearlescent pigmentcomprises the steps of: providing a platelet pigment; increasing themagnetic mass susceptibility; wherein the color difference (ΔE*) betweenthe provided pigment and the resultant pigment is not more than about 5.

In another embodiment, the pearlescent pigment formed by the stepscomprising: providing a platelet pigment; increasing the magnetic masssusceptibility; wherein the color difference (ΔE*) between the providedpigment and the resultant pigment is not more than about 5.

The method may be used to transform conventional, metallic-shade,low-magnetic Fe₂O₃ coated pearlescent pigments into highly magneticpigments without changing their apparent color. This expands theavailable color range of iron-oxide coated magnetic pigments beyond darkcolors, such as black or brown, or dark shades with muted interferencecolors, such as Colorona® Blackstar Blue, Red, Green, and Gold. Based onthe process described, traditional bronze, copper, russet and othertraditional metallic pearlescent pigments can be applied in magneticallyaligned coatings.

In addition, the color difference between the non-magnetic startingmaterial and the magnetic product cannot easily be distinguished by thehuman eye. This allows for interesting and cost-effective stylingopportunities involving both pigments. For example in automotiveapplications, the described magnetic pearlescent pigment may be used forstyling accents, such as three-dimensional emblems, logos, designs, etc,that utilize the magnetic properties of the pigment while the balance ofthe automotive body may be coated via traditional processes usingnon-magnetic pearlescent pigments having the same color.

Color matched magnetic and non-magnetic pearlescent pigments may be usedin advanced segmented displays based on combinations of the two pigments(dry pigment or un-cured liquid based displays). The two pigments wouldhave equivalent color and be indistinguishable so there would be anundetectable transition between the display segments (containing themagnetic pigments) and the display background (containing thenon-magnetic pigments). The display segments and background may containthe non-magnetic and magnetic pigments, respectively. By applying amagnetic field, the magnetic pigments would be re-oriented so they wouldhave a different color than the non-magnetic pigments. Applying amagnetic field with a different orientation would allow the magneticpigments to have the same color as the non-magnetic pigments. Theorientation of the magnetic pigments may persist without applying amagnetic field allowing a more energy efficient display. Anotherpotential application of the color matched magnetic and non-magneticpigments is in the areas of Security and Brand Protection. 3D effectsmay be created by preferential orientation of the magnetic flakes incertain directions following a certain shape set by one or more magneticfields during or right after an ink or coating containing the magneticflakes has been applied onto a secure document. Such features are almostimpossible to reproduce with traditional graphic arts inks. In addition,it is possible to print onto an article a hidden magnetic pattern forexample in the form of an image or a code. This hidden image could beread (or authenticated) by a magnetic probe or reader going over thedocument. Examples of articles may include checks, secure documents suchas passports, driver's licenses, ID cards, or credit cards.

The magnetic susceptibility of α-Fe₂O₃ coated pearlescent pigments maybe increased significantly with only small or no change in the overallcolor (both dry pigment color and the appearance of the pigment incoatings). Magnetic pigments may be produced by the described methodswherein the color of its corresponding coating in regions of maximumplatelet orientation normal to the plane of the coating, controlled byan external magnetic field, can be black (Fe₃O₄ surface coated) or red(γ-Fe₂O₃ coated).

In order to improve the light, water repellency, weather stability,texture, and dispersion ability, it is frequently advisable to subjectthe finished pigment to surface treatment, depending on the area ofapplication. Examples of surface treatments are methicone(poly(oxy(methylsilylene))), metal soap, fatty acid, hydrogenatedlecithin, dimethicone (polydimethylsiloxane), fluorinated compounds,amino acids, N-acylamino acids, glyceryl rosinates, silanes, andcombinations. Many of the processes are described in U.S. Pat. Nos.6,790,452; 5,368,639; 5,326,392; 5,486,631; 4,606,914; 4,622,074;5,759,255; 5,759,255; 5,571,851; 5,472,491; 4,544,415; and 5,759,255;German Patent 22 15 191; DE-A 31 51 354; DE-A 3235017; DE-A 3334598; DE4030727A1; EP 0 649 886 A2; WO 97/29059; WO 99/57204; EP 0090259; EP 0634 459; WO 99/57204; WO 96/32446; WO 99/57204; WO 01/92425; J. J.Ponjee, Philips Technical Review, Vol. 44, No. 3, 81 ff; and P. H.Harding J. C. Berg, J. Adhesion Sci. Technol. Vol. 11 No. 4, pp.471-493. This post-coating may further increase the chemical stabilityor simplify handling of the pigment, in particular incorporation intovarious media. In order to improve the wettability, dispersibilityand/or compatibility with the user media, functional coatings of Al₂O₃or ZrO₂ or mixtures thereof may be applied to the pigment surface.

In one embodiment, coupling agents may be used to form an outer layer onthe pearlescent pigment. Suitable coupling agents are disclosed in EP632 109. Examples include, silanes, zirconium aluminates, zirconates,and titanates. The silanes may possess the structure Y—(CH₂)n-SiX₃ inwhich n is 2-18, Y is an organofunctional group, e.g. an amino,methacrylic, vinyl, alkyl, aryl, halogen and/or epoxy group, and X is asilicon-functional group which following its hydrolysis reacts withactive sites of an inorganic substrate or by condensation with othersilicon compounds. This group Y may comprise, for example a hydroxy, ahalogen or an alkoxy group.

In addition to these substantially hydrophilic coupling agents, it isalso possible to use hydrophobic silanes, especially the aryl-, alkyl-and fluoroalkyl-substituted di- and trimethoxysilanes. These include,for example, phenethyltrimethoxysilane, propyltrimethoxysilane,butyltrimethoxysilane, isobutyltrimethoxysilane, hexyltrimethoxysilane,octyltrimethoxysilane, 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane and(3,3,3-trifluoropropyl)methyldimethoxysilane. The concentration ofcoupling agents may be 0.2-5% by weight with respect to the basepigment.

In one embodiment a cosmetic composition contains the pigment. Thecosmetic composition may be useful for make-up products for the skin,the eyes, or hair. Examples of compositions intended as make-up for theskin include eye shadows, eye liners, mascaras, body or face powder,foundations, blushes, colored creams, nail polish, lipsticks, lip gloss,hair or body gel, hair or body wash, cover sticks, lotion, concealer,foundation, and anti-aging cream. Examples of cosmetic applicationsinvolving the lip area, are lip gloss, lipstick, and other lipcompositions. Nail polish may be referred to as nail varnish, or nailenamel. Other examples of cosmetic compositions include hair coloringcompositions, shampoos, skin cream, skin treatment products such asscrubbing, exfoliating, cleansing or acne treatment gels, creams orlotions.

Pearlescent pigments may be used to produce a makeup cosmetic asdescribed in U.S. Pat. No. 6,663,852, U.S. Pat. No. 6,451,294, and6280714.

The pigment may be applied in a dry form or in combination withbinder/additive of sufficient quantity for cosmetic applications. Thesecosmetics may allow one to create high intensity colors.

In one embodiment, the pigments of the composition are aligned during orafter application of the composition. An example of aligning thepigments of the composition is by applied the composition with amagnetic applicator. The magnetic applicator may be used to align themagnetic particles in the cosmetic allowing control of their appearance.

General cosmetic compositions may contain preservatives, stabilizers,neutralizing agents, aqueous-phase thickeners (polysaccharidebiopolymers, synthetic polymers) or fatty-phase thickeners, such as clayminerals, fillers, perfumes, hydrophilic or lipophilic activesubstances, surfactants, antioxidants, film-forming polymers andmixtures thereof. The amounts of these various ingredients are thoseconventionally employed in the fields in question and, for example, maybe from 0.01 to 30% of the total weight of the composition. In oneembodiment, the cosmetic composition may further comprise a binderwherein the pigment represents about 0.5% to about 99.5% of thecomposition.

Lip cosmetic composition may comprise any ingredient usually used in thefield concerned, such as water, preferably in an amount ranging from 0to 95% of the total weight of the composition, water-soluble orliposoluble dyes, antioxidants, essential oils, preserving agents,fragrances, neutralizing agents, liposoluble polymers, in particularhydrocarbon-based polymers such as polyalkylenes or polyvinyl laurate,gelling agents for an aqueous phase, gelling agents for a liquid fattyphase, waxes, gums, surfactants, additional cosmetic or dermatologicalactive agents such as, for example, emollients, moisturizers (forexample glycerol), vitamins, liquid lanolin, essential fatty acids,lipophilic or hydrophilic sunscreens, and mixtures thereof. Thecomposition may also contain lipid vesicles of ionic and/or nonionictype. These ingredients (other than the water) may be present in thecomposition in a proportion of from 0 to 20% of the total weight of thecomposition.

In one embodiment a composition or article comprises the pigment. Thecomposition may be a coating, ink, plastic, or paint. A coating, ink,plastic, or paint may further comprise a binder, wherein the pigmentrepresents about 0.5% to about 99.5% of the composition, about 0.1% toabout 70%, or about 0.2% to about 10%. In one embodiment a compositionor article may additionally comprise a pigment with a low or no magneticsusceptibility. In another embodiment an article comprises thepearlescent pigment.

The coating, ink, plastic, or paint may be printing ink, surfacecoating, coatings for laser marking, pigment preparation, drypreparation, food colorant, automotive coating, refinish coating,textile coating, architectural coating, synthetic fiber, or fiber basedproduct. A coating may be applied to an object as a liquid, vapor, orsolid. Examples of methods for applying a coating are by printing,painting, polymeric coating, or spraying. The coating may be a powder,enamel, aerosol, paint, epoxy, or polymer. Additional examples of usesfor coating, ink, plastic, or paint are for uses in preventingcounterfeiting, such as for secure documents, passport, currency,checks, credit cards and driver licenses; brand protection applications;agricultural, such as seed and mulch coloring; textile coloring; andfood applications.

In one embodiment a coating contains the pigment. Examples of uses forthe paint may be: industrial, automotive, consumer electronics andarchitectural. Examples of automotive applications are: OEM, refinish orspecialty (custom) automotive applications.

The ink may be a magnetic toner. An example of a magnetic toner is onethat is used for Magnetic Ink Character Recognition (MICR). These tonersmay be used to print security codes on checks and are read by low-costreaders. Many of the toners used for MICR are black. The color andmagnetic susceptibility of the MICR ink may be adjusted by usingdifferent pearlescent pigments.

The arts of making coatings and inks, as well the various printingprocesses (i.e., intaglio, flexo, screen, offset, gravure) are very wellknown in the literatures, so it is not repeated here [see “The PrintingInk Manual”, 5^(th) edition, R. H. Leach, ed. Taylor & Francis, Inc.].Other less common printing processes include digital offset solutionssuch as the Hewlett-Packard Indigo presses.

Besides the topical applications such as printings or coatings, thepigments can be incorporated directly into substrates during theformation stage to make an article. For example, into paper, thepigments can be introduced along with other regular paper fillers suchas calcite, talc during paper making to fill the open pores of papernear the surface. If the article is a plastic, the pigment can beintroduced during the extrusion of substrate. Examples of articles areplastic, glass, ceramic material, concrete, pressed wood, pills paper,toothpaste, candles, food products, or agricultural products. Otherapplications where pearlescent pigments may be used include householdproducts such as detergents and cleaning products.

The terms goniochromatic, iridescent, and pearlescent, may be usedinterchangeably to mean a change of color depending on the viewingangle.

While the present disclosure has illustrated by description severalembodiments and while the illustrative embodiments have been describedin considerable detail, it is not the intention of the applicant torestrict or in any way limit the scope of the claims to such detail.Additional advantages and modifications may readily appear to thoseskilled in the art.

EXAMPLES Example 1 Magnetic Bronze Pearlescent Pigments

Catalyst Preparation:

Anhydrous ethylene glycol (320 g) and K15 polyvinylpyrrolidone (40 g)were mixed at 3000 rpm with a Hauschild mixer until dissolved. Themixture was then added to a IL PTFE-lined cylindrical reactor with a 2″PTFE coated 3-leaf agitator blade and a nitrogen purging line. In aseparate beaker, anhydrous ethylene glycol (320 g) and hydratedchloroplatinic acid crystal (2 g, H₂PtCl₆-6H₂O) were stirred untilhomogeneous. The ethylene glycol mixture was then sonicated for 10minutes to remove oxygen and then charged into the reaction vesselcontaining the PVP solution. Additional anhydrous ethylene glycol (320g) was added to the reaction vessel and agitated at approximately 200rpm under room temperature conditions. A nitrogen purging line was thenlowered to just below the liquid surface to provide an inert atmosphere.The mixture was then heated from approximately 20° C. to 120° C. inroughly 100 minutes. After one hour at 120° C., the Pt-in-(PVP/EG)liquid (0.75 mg Pt per g fluid] was cooled, poured into a glass jar andsealed. Highly active, PVP stabilized platinum nanoparticles with a meanparticle size in the range of 2-10 nm was obtained.

Reduction:

Fe₂O₃ coated natural mica pigment (20 g, SunPearl Bronze (compositionshown in Table 1)) and PVP stabilized Pt catalyst (4 g) in ethyleneglycol were dispersed in polyethylene glycol 400 (96 g) and added to a600 mL steel Parr reactor equipped with twin 45 degree pitch bladeimpellers. Agitation was maintained at approximately 800 rpm. Thereaction solution was purged several times by pressurizing the vesselwith nitrogen then evacuating under vacuum. Following sufficientpurging, the mixture was heated to 220° C., pressurized with hydrogen to10.3 bar and held at these conditions for 6 hours. The pigment wasfiltered, rinsed with deionized water (4 L), ethanol (1 L), and dried at60-80° C. A deep and intensely colored, golden-beige magneticpearlescent pigment comprised of natural mica containing an inner layerof α-Fe₂O₃ (hematite) and a surface layer of Fe₃O₄ (magnetite) wasobtained (referred to as Example 1a).

Oxidation:

The golden-beige pigment produced in Example 1a (3.5 g) was heated inair at 350° C. for 45 minutes using a Barnstead Thermolyne Model 1400box furnace. Calcination transformed the outer magnetite layer intomaghemite (γ-Fe₂O₃) resulting in a lustrous, bronze-colored magneticpearlescent pigment (Example 1b).

Pigment Drawdown:

Pigment (SunPearl Bronze, Example 1a, and Example 1b) drawdowns wereprepared by dispersing 0.5 g of pigment in 4.5 g of Delstar DMR499acrylic enamel at 3000 rpm for 3 minutes using a DAC150FVZ-K model(Hauschild Engineering) high speed mixer. The pigment suspensions werethen applied to a plain white card (BYK Gardner, AG-5142) using a 3 mil(˜76 micron) Bird applicator.

Magnetic Alignment:

Circular button magnets (13 mm ProMAG® Neodymium (Grade 35, 12,300gauss) magnet) were placed beneath a 0.32 cm thick glass plate. Afterapplication of the pigment suspension and prior to curing, the card wasplaced on the glass plate such that a circular magnet is locateddirectly beneath a selected portion of each coating.

Upon placement of the card, the pigment prepared in Examples 1a and 1binstantaneously oriented into a three-dimensional circular pattern withunique depth of perception. After 10 to 15 minutes in an oven held at50° C., the three-dimensional image was cured and fixed within thecoating.

Color Analysis

CIELab values for all pigments prepared, their corresponding startingpigment, and the magnetically aligned pigments were measured with aSpectraflash SF600 Plus spectrophotometer (9 mm aperture), see Table 2.The magnetically aligned pigments were measured in the center portion ofthe aligned circular image, the region of highest magnetic force. Thealigned pigments display a dramatic color shift induced upon applicationof the magnetic field.

After hydrogenation, Example 1a has a lower L value (ΔL*=−9.32), a value(Δa*=−12.68), and b value (Δb*=−12.65), relative to the startingsubstrate resulting in an overall color difference (ΔE*) of 20.19 (seeTable 2). The reduction results in an increase in the magnetic masssusceptibility of the starting substrate from 0.019×10⁻⁵ m³/kg to5.056×10⁻⁵ m³/kg (measured using a Bartington MS2 susceptibility meter).

The increase of the magnetic susceptibility allows the platelet-likepigments to be oriented by an applied magnetic field. As shown in Table2, application of a magnetic field to uncured coatings containing thepigment produced in Example 1a results in the appearance of athree-dimensional circular image with interesting depth and a very darkblack appearance (L*=29.75 and C*=5.33) in the region of maximumplatelet alignment.

Oxidation of the pigment produced in Example 1a transforms the magnetite(Fe₃O₄) surface coating to maghemite (γ-Fe₂O₃) resulting in apearlescent pigment comprising a natural mica substrate with an innerlayer of α-Fe₂O₃ (hematite) and a surface layer of γ-Fe₂O₃. The γ-Fe₂O₃retains the magnetic properties gained in the reduction step andrestores the color of the original substrate (ΔE*=1.62). As shown inTable 2, application of a magnetic field to uncured coatings containingthe pigment produced in Example 1b results in the appearance of athree-dimensional circular image with interesting depth and a highchroma red appearance (L*=37.28, C*=30.92, hue angle=34.41) in the areaof maximum platelet orientation.

The ΔE* may be further reduced by optimization of the process conditionsused in reduction and oxidation for a given substrate.

TABLE 1 Composition of the substrates (supplied by SunChemical) used inExamples 1-3 Product Name Mica (%) α-Fe₂O₃ (%) Particle Size RangeSunPearl Bronze 65-69 31-35 10 to 60 μm SunPearl Copper 62-66 34-38 10to 60 μm SunPearl Maroon 57-61 39-43 10 to 60 μm

TABLE 2 CIELab values measured for Examples 1-3 using a 10° observer andD65 illuminant with specular component included (9 mm aperture). Colordifference (ΔE*) values are measured relative to SunPearl Bronze, Copperand Maroon for Examples 1, 2 and 3, respectively. Dry Pigment ExampleSample Color L* A* B* C* Hue angle ΔE* 1 SunPearl Bronze 62.47 19.0228.24 34.05 56.04 Bronze Example 1a Golden- 53.15 6.34 15.59 16.83 67.8720.19 Beige 3-D Circular 29.75 3.50 4.01 5.33 48.89 Image in Example 1aExample 1b Bronze 62.00 17.58 27.67 32.78 57.57 1.62 3-D Circular 37.2825.51 17.47 30.92 34.41 Image in Example 1b 2 SunPearl Copper 52.6330.48 27.09 40.78 41.63 Copper Example 2a Dark 49.72 23.97 21.69 32.3342.13 8.95 Copper 3-D Circular 37.07 17.93 13.27 22.31 36.49 Image inExample 2a Example 2b Copper 52.05 28.88 24.93 38.15 40.79 2.75 3-DCircular 36.56 23.98 15.74 28.68 33.28 Image in Example 2b 3 SunPearlMaroon 45.89 32.99 16.54 36.91 26.64 Maroon Example 3a Dark 44.17 27.8314.09 31.20 26.85 5.96 Maroon 3-D Circular 36.01 21.87 10.66 24.33 25.99Image in Example 3a Example 3b Maroon 46.26 32.91 16.87 36.98 27.13 0.503-D Circular 42.73 30.14 14.90 33.63 26.31 Image in Example 3b

Examples 2 and 3 Magnetic Copper and Maroon Pearlescent Pigments

Pearlescent pigment were produced in the same manner as Example 1 exceptother α-Fe₂O₃ coated natural mica metallic shades used were, SunPearlCopper (SunChemical) and SunPearl Maroon (SunChemical). The reduced andre-oxidized shades of SunPearl Copper were Examples 2a and 2b,respectively, while the reduced and re-oxidized shades of SunPearlMaroon were 3a and 3b, respectively.

The drawdown procedure used in Example 1 was applied to the pigmentsprepared in Examples 2 and 3. CIELab values measured for Examples 2a,2b, 3a and 3b, as well as in the center portion of the aligned circularimages within 2a, 2b, 3a and 3b are shown in Table 2.

The pigments of Examples 1b, 2b, and 3b have different colors becausethey have different amounts of α-Fe₂O₃ and γ-Fe₂O₃ layer(Bronze<Copper<Maroon), see Table 3. The magnetic susceptibility ofExample 1b was higher than Example 2b, which was higher than Example 3bas shown in Table 4. The trend in magnetic susceptibility(Bronze>Copper>Maroon) may be attributed to the initial α-Fe₂O₃ contentof the starting material. Given that each substrate was subjected to thesame reduction and oxidation treatments, the overall ratio of magnetiteor maghemite present in the outer layer to hematite present in the innerlayer was higher for Example 1b relative Example 2b, which was higherrelative to Example 3b.

Due to different magnetic properties of each sample, the amount ofalignment associated with each pigment was slightly different. Pigmentsof Example 1a and 1b are had a sharp transition between aligned regionsexposed to the magnetic field and the regions not exposed to themagnetic field. This sharp transition results in a large region ofpigment particles aligned normal to the coating surface yielding a blackappearance for Example 1a (magnetite coated) and a red appearance forExample 1b (maghemite coated). Pigments of Example 2 had a slightly moresubtle transition giving the appearance of a three dimensional circularhole with less depth than the pigments of Example 1. Only the verycenter of the circular image of aligned pigments of Example 2a had adark black appearance. Only the very center of the circular image ofaligned pigments of Example 2b had the red absorbance color ofmaghemite. Pigments of Example 3 had an even more subtle transition thanthe pigments of Example 2, giving the appearance of a three dimensionalcircular hole with less depth than the pigments of Example 2.

TABLE 3 Composition of Pigments Sample mica α-Fe₂O₃ γ-Fe₂O₃ Fe₃O₄Example 1a 66.5% 26.0% 0.0% 7.5% Example 1b 67.4% 24.7% 7.7% 0.1%Example 2a 61.2% 35.6% 0.0% 3.2% Example 2b 61.4% 35.0% 3.3% 0.2%Example 3a 58.8% 38.4% 0.0% 2.9% Example 3b 59.4% 37.5% 3.0% 0.1%

TABLE 4 Magnetic Susceptibility Magnetic Susceptibility Example Sample(m³/kg) 1 SunPearl Bronze 0.019 × 10⁻⁵ Example 1a 5.056 × 10⁻⁵ Example1b 5.204 × 10⁻⁵ 2 SunPearl Copper 0.015 × 10⁻⁵ Example 2a 1.213 × 10⁻⁵Example 2b 0.984 × 10⁻⁵ 3 SunPearl Maroon 0.010 × 10⁻⁵ Example 3a 0.785× 10⁻⁵ Example 3b 0.792 × 10⁻⁵

Example 4 High Chroma Magnetic Orange Pearlescent Pigment (10-60 micron)

A solution containing 706.2 g of 0.1 M HCl, 33.3 g of 38.4 wt % FeCl₃solution, 192 g prilled urea and 40 g of pigment from Example 1b wascharged into a 1 L jacketed pot reactor under agitation at 180 rpm. Thisinitial solution had an approximate pH of 1.8. The solution was thenheated to 90° C. to promote the decomposition of urea and a subsequentrise in pH. After about 1 to 2 hours at 90° C., the solution pH rose toapproximately 6.3-6.5 indicating completion of the reaction. For workup, the pigment was filtered, rinsed with water, and dried at 65° C. Anintensely colored, lustrous magnetic orange pearlescent pigment wasobtained.

1. A pigment comprising a substrate and a layer, wherein the layer hasregions of γ-Fe₂O₃ and α-Fe₂O₃.
 2. The pigment of claim 1, wherein thesubstrate is a platelet pigment.
 3. The pigment of claim 1, wherein theregion of γ-Fe₂O₃ is further from the substrate than the region ofα-Fe₂O₃.
 4. The pigment of claim 1, wherein the pigment comprises asubstrate selected from the group consisting of natural mica, syntheticmica, glass flakes, metal flakes, talc, kaolin, Al₂O₃ platelets, SiO₂platelets, TiO₂ platelets, graphite platelet, BiOCl, calciumborosilicate, synthetic alumina, and boron nitride.
 5. The pigment ofclaim 1, wherein located between the substrate and the layer is one ormore layers selected from the group consisting of TiO₂, Fe₂O₃, FeOOH,ZrO₂, SnO₂, Cr₂O₃, BiOCl, and ZnO.
 6. The pigment of claim 3, whereinthe mean thickness of the layer is about 10 nm to about 350 nm.
 7. Thepigment of claim 1, wherein the α-Fe₂O₃ to γ-Fe₂O₃ ratio is between 0.05and
 50. 8. The pigment of claim 1, wherein the pigment has a magneticsusceptibility of about 0.1×10⁻⁵ to 15×10⁻⁵ m³/kg.
 9. The pigment ofclaim 1, wherein the pigment additionally comprises a layer of a metaloxide selected from the group consisting of: TiO₂, FeOOH, ZrO₂, SnO₂,Cr₂O₃, BiOCl, and ZnO, where the additional layer is adjacent to theoutside surface of the layer containing the α-Fe₂O₃ and γ-Fe₂O₃.
 10. Thepigment of claim 1, wherein the substrate is a fiber.
 11. A process forenhancing the magnetic properties of pigments comprising the stepsof: 1) providing a substrate with an layer comprising Fe₂O₃, with amagnetic susceptibility less than 0.1×10⁻⁵ m³/kg; 2) reducing some orall of the Fe₂O₃ to Fe₃O₄; 3) subsequently oxidizing some or all of theFe₃O₄ to γ-Fe₂O₃.
 12. The process of claim 11, wherein the Fe₂O₃ in thefirst step is partially or completely comprised of α-Fe₂O₃.
 13. Theprocess of claim 11, wherein the Fe₂O₃ is reduced by a hydrogen source.14. The process of claim 11, wherein the Fe₃O₄ is oxidized by heatingthe pigment to greater than about 350° C. in an oxidizing atmosphere.15. The process of claim 11, wherein the pigment comprises a substrateselected from the group consisting of natural mica, synthetic mica,glass flakes, metal flakes, talc, kaolin, Al₂O₃ platelets, SiO₂platelets, TiO₂ platelets, graphite platelet, BiOCl, calciumborosilicate, synthetic alumina, and boron nitride.
 16. The process ofclaim 15, wherein located between the substrate and the iron oxide layeris one or more layers selected from the group consisting of TiO₂, Fe₂O₃,FeOOH, ZrO₂, SnO₂, Cr₂O₃, BiOCl, and ZnO.
 17. The process of claim 11,wherein after oxidation the pigment has a magnetic susceptibility ofabout 0.1×10⁻⁵ to 15×10⁻⁵ m³/kg.
 18. The process of claim 11, whereinthe color difference (ΔE*) of the pigment prior to reduction and afteroxidation is not more than about
 5. 19. A pigment formed by the stepscomprising: 1) providing a platelet pigment; 2) increasing the magneticmass susceptibility; wherein the color difference (ΔE*) between theprovided pigment and the resultant pigment of step 2 is not more thanabout
 5. 20. The pigment of claim 19, wherein resultant pigment of step2 has an α-Fe₂O₃ to γ-Fe₂O₃ ratio is between about 0.05 and about 50.21. The pigment of claim 19, wherein the resultant pigment of step 2 hasa magnetic susceptibility of about 0.1×10⁻⁵ to about 15×10⁻⁵ m³/kg. 22.A composition or article comprising the resultant pigment of step 2 ofclaim 19, wherein the composition is selected from the group consistingof a coating, ink, plastic or paint.
 23. The composition or article ofclaim 22, wherein the composition or article additionally comprises theprovided pigment in step
 1. 24. The coating of claim 22, wherein thecoating is used in automotive applications.